If you spend a significant amount of time working with any type of scripting, code, or markup, then you’re probably looking at a monospace (fixed-width for each character) font.
The quality of these fonts varies, though the defaults that ship with Mac OS X, Windows XP, and Windows Vista are quite good. The Consolas font included with Vista is particularly good.
Fortunately, there is a quality free/open alternative. Raph Levien has developed a great programming font cleverly called Inconsolata. I have been using it as my primary terminal/coding/text font for several months and find it superior to anything else I’ve used.
Here’s a quick screenshot of Inconsolata used in a simple PHP file on my desktop.